Nonlinear Frequency Conversion (NLFC) is a widely used optical technique for generating specific wavelengths using laser devices. In a NLFC device a light beam, with a fundamental wavelength, enters a NLFC component which converts some of the light with the fundamental wavelength into light with another wavelength. The most common variation of this technique uses a light beam with fundamental wavelength that is frequency-doubled resulting in a converted beam with half the fundamental wavelength, a process known as second harmonic generation (SHG). The NLFC process does not convert all photons of the fundamental beam, however, leading to a spatial overlap of the converted beam with the fundamental beam exiting the NLFC component.
Many applications require only the converted beam so that the fundamental beam exiting the component must be removed. In the specific wavelength range of ultra-violet (UV) radiation between 200 nm and 270 nm, there are different methods found in the prior art to remove the fundamental beam in NLFC devices by placing an element at some point in the beam path after the NLFC component.
An interference filter with a multilayer coating which has high reflectivity for the fundamental beam and transmits a fraction of the converted beam is disclosed in U.S. Pat. No. 7,110,426, Rudolph [issued Sep. 19, 2006, U.S. Army Research Contract W911NF-09-1-0102], and Nishimura et al. [Japanese Journal of Applied Physics 42, 5079, (2003)]. The filter must be made of a UV transparent material such as UV fused silica which is expensive. Furthermore, the performance of those interference filters is not very high for UV wavelengths. In particular, the transmission efficiency of the filters can be as low as 90% (where the transmission efficiency of the filter is defined as the ratio of the power of the converted beam which is transmitted through the filter divided by the power of the converted beam incident on the filter) so there is a disadvantageous loss of output power of the converted beam. In addition, the rejection ratio of the filters can be low (where the rejection ratio is defined as the ratio of the power of the fundamental beam which is incident on the filter divided by the power of the fundamental beam which is transmitted through the filter), so an additional beam separating element can also be needed to further reduce the power of the fundamental beam.
Prisms are used to separate the beams in Tangtronbenchasil et al. [Japanese Journal of Applied Physics 45, 6315, (2006)], and Ruhnke et al. [Optics Letters 40, 2127, (2015)]. The use of a prism makes the laser devices bulky and heavy because a long beam path through the prism is required to ensure good separation between the fundamental and converted beams. The requirement of UV transmittance makes the prism expensive and not suitable for low cost devices.
Frequency-doubling devices can be categorised depending on the polarisation properties of the fundamental and converted beam. In “type I” devices the linearly polarised converted beam exiting the component has an orthogonal polarisation to the linearly polarised fundamental beam. The 90° change in polarisation can be exploited to separate the fundamental and converted beams by using Brewster mirror reflection as in U.S. Pat. No. 8,559,471 (Mao, issued Oct. 15, 2013). A mirror which has high reflectivity to the converted beam and transmits the majority of the fundamental beam is oriented at Brewster angle in a device described by Tangtrongbenchasil et al. [Japanese Journal of Applied Physics 47, 2137, (2008)]. Other features for frequency-doubled lasers capable of emitting deep ultraviolet light are disclosed in U.S. Pat. No. 8,743,922B2 (Smeeton et al., issued Jan. 3, 2014) and U.S. Pat. Pub. No. 2015/0177593A1 (Smeeton et al., published Jun. 25, 2015).